1. Field of the Invention
The present invention relates to semiconductor devices such as field effect transistors composed of GaN-based compound semiconductor.
2. Background Art
GaN-based compound semiconductors such as GaN, InGaN, AlGaN, and AlInGaN have a higher bandgap energy than other Group III-V compound semiconductors, e.g., GaAs-based compound semiconductors. Therefore, electronic devices formed using GaN-based compound semiconductor material have resistance to high temperatures and exhibit excellent operating characteristics at elevated temperatures. Research has been conducted in recent years on high electron mobility transistors composed of GaN-based compound semiconductor.
Such high electron mobility transistors are disadvantageous in that the SiC substrate and the GaN-based compound semiconductor layer formed thereon have a large difference in lattice constant, resulting in many defects in the semiconductor layer. Charge is trapped in these defects and at the interface between the semiconductor layer and the overlying passivation film and later released. This causes hysteresis in the source-drain current vs. voltage characteristic of the high electron mobility transistor when the transistor operates at high voltage and high power levels, resulting in a decrease in the drain current, a phenomenon called current collapse. If such a transistor is used in a system that receives varying power, there will be a temporary drop, or slump, in the output power of the system.
In order to overcome this problem, a semiconductor device has been proposed in which a luminescent layer is provided on the top surface side or bottom surface side of the transistor so that trapped charge is released and recombined by the light generated by the luminescent layer (see, e.g., Japanese Laid-Open Patent Publication No. 2006-286746).